1. Field of the Invention
This invention relates to radiation detection. More particularly, the invention relates to a method and apparatus for active detection of fissile material with some particular applications in homeland security.
2. Description of the Related Art
Active Interrogation Overview
The detection and characterization of highly enriched uranium (HEU) is desired for monitoring and control, treaty verification and homeland security applications. The passive detection of radiation naturally emitted by HEU is not efficient, because the neutron yield is minute, and gamma rays are of low energy and can be shielded.
Active interrogation methods have been developed and used for some time. For example, see Moss, C. E., Goulding, C. A., Hollas, C. L., & Myers, W. L. (2004). “Neutron Detectors for Active Interrogation of Highly Enriched Uranium.” IEEE TRANSACTIONS ON NUCLEAR SCIENCE, 51 (4), 1677-1681. They employ pulsed sources that direct an interrogating beam of neutrons or gammas to the suspect HEU material, which in response emits secondary radiations, typically neutrons from fission. Appropriate detectors are then used to count the delayed fission neutrons.
A possible setup is illustrated in FIG. 1, where a linear accelerator (linac) is used to produce bremsstrahlung photons irradiating a cargo container. The photons will induce fission in any HEU material present inside the container, and a detector positioned nearby will detect fission neutrons emitted by the HEU. The number of neutrons counted depends on a number of parameters, including the amount of HEU present, its distance from the detector, size of the detector, the presence of shielding materials, and the measurement time.
Neutron Detectors
The detector requirements for active interrogation of HEU include high detection efficiency for neutron energies around 1 MeV, efficient gamma discrimination, and relatively large sizes, as required in cargo screening applications, for instance. Smaller or portable detectors are also useful when close proximity to the HEU is possible.
Neutron detectors based on pressurized 3He gas satisfy the above requirements and are commonly employed in passive detection, as in portal monitoring. In those applications, 3He tubes are embedded in moderating material (polyethylene), as pictured in FIG. 2, that thermalizes the high energy neutrons to achieve good detection efficiency. The same configuration can be applied to the detection of 1 MeV fission neutrons, in active interrogation setups. Here, thermal neutron shielding, such as cadmium, must be installed all around the detector assembly, to prevent counting of neutrons that have thermalized in the ground and surrounding structures. A typical response of this detector assembly is shown in FIG. 3. The recorded rate is significantly higher than the background rate, revealing the presence of HEU.
Recovery Time.
It is shown in FIG. 3 that the useful part of the detector response begins after ˜3 ms. Prior to that, the detector and electronics are busy counting gamma rays generated by the linac pulse, even though the pulse itself has a period of under 10 μs. The large delay observed in the recovery of the detector response is in part due to the slow drift of ions inside the 3He tubes.
3He Inventory and Production.
In addition to the timing limitations, neutron detectors based on 3He gas cannot support large deployments, due to the isotope's very limited availability on Earth. The diminishing inventory and minute natural abundance of 3He gas necessitate the adoption of new technologies for the detection of neutrons, especially in homeland security and international safeguards applications, where large volume deployments requiring many kilograms of 3He are required. The only practical source of 3He on Earth is through production of the intermediary radioactive tritium (3H) gas. Tritium decays to 3He at a rate of 5.5% per year. Tritium was produced over the time frame from 1955 to 1988 for use as a critical ingredient of nuclear weapons. Production ceased in the US in 1988 and likely will not resume, as there is currently an adequate supply to sustain the diminishing nuclear weapons inventory.
Despite the low and dwindling 3He supply, no attractive alternate neutron detector has been successfully identified for large detectors that must have very low gamma sensitivity and low cost. The US Department of Homeland Security (DHS) and Customs and Border Protection (CBP) plan to equip major US ports of entry with large area neutron detectors, in an effort to intercept the smuggling of nuclear materials, potentially used in terrorist attacks. It is estimated that the annual demand of 3He for US security applications alone is 22 kiloliters, more than the worldwide supply. See R. L. Kouzes, “The 3He supply problem,” PNNL report 18388, April 2009.
This is strongly impacting science applications of 3He at neutron scattering facilities, where planned 3He detector installations require as many as 20 kiloliters per year worldwide. See Helium detector expert group, “The 3Helium supply crisis and alternative techniques to 3Helium based neutron detectors for neutron scattering applications,” Proceedings of meeting held at FRM II, Munich, July 2009, available online at http://cstsp.aaas.org/Helium3/He3%20 Minutes-FRM-II.doc. Safeguards applications, including treaty verification and nuclear waste characterization, demand an additional 20 kiloliters yearly. All in all, we estimate that the projected total 3He deficit is more than 60 kiloliters annually. Clearly, alternate neutron detection technologies must be adopted in order to accommodate the rising demand for detectors in the setting of dwindling supply of 3He.
The background to the present invention and related art is best understood by reference to Applicant's own prior work, including in particularly, U.S. Pat. No. 7,002,159 B2 (the '159) entitled “Boron Coated Straw Neutron Detector” which issued Feb. 21, 2006. The '159 is hereby incorporated by reference in its entirety, for all purposes, including, but not limited to, supplying background and enabling those skilled in the art to understand, make and use in Applicant's present invention.
Applicant's other issued patents and pending applications may also be relevant, including; (1) U.S. Pat. No. 5,573,747 entitled, “Method for Preparing a Physiological Isotonic Pet Radiopharmaceutical of 62CU; (2) U.S. Pat. No. 6,078,039 entitled, “Segmental Tube Array High Pressure Gas Proportional Detector for Nuclear Medicine Imaging”; (3) U.S. Pat. No. 6,264,597 entitled, “Intravascular Radiotherapy Employing a Safe Liquid Suspended Short-Lived Source”; (4) U.S. Pat. No. 6,483,114 D1 entitled, “Positron Camera”; (5) U.S. Pat. No. 6,486,468 entitled, “High Resolution, High Pressure Xenon Gamma Rays Spectroscopy Using Primary and Stimulated Light Emissions”; (6) U.S. Pat. No. 7,078,704 entitled, “Cylindrical Ionization Detector with a Resistive Cathode and External Readout”; (7) U.S. patent application Ser. No. 10/571,202, entitled, “Miniaturized 62Zn/62CU Generator for High Concentration and Clinical Deliveries of 62CU Kit Formulation for the Facile Preparation of Radiolabeled Cu-bis(thiosemicarbazone) Compound”; U.S. patent application Ser. No. 12/483,771 entitled “Long Range Neutron-Gamma Point Source Detection and Imaging Using Rotating Detector”; U.S. patent application No. 61/183,106 entitled “Optimized Detection of Fission Neutrons Using Boron Coated Straw Detectors Distributed in Moderator Material. Each of these listed patents and patent applications are hereby incorporated by reference in their entirety for all purposes.